Electrochemical treatment of ferrous metal

ABSTRACT

A TWO-STEP ELECTROCHEMICAL TREATMENT OF A FERROUS METAL ARTICLE IN WHICH A COATING OF ELECTROLYTIC CHROMIUM PLATE IS DEPOSITED ON A FERROUS METAL SURFACE AND A SURFACE COATING CONTAINING CHROMIUM AS AN OXIDE OF CHROMIUM AND METALLIC CHROMIUM IS CODEPOSITED OVER THE CHROMIUM PLATE FROM A CHROMIC ACID SOLUTION CONTAINING AN ELECTROPLATING ADDITIVE, PREFERABLY A HALIDE ION. BY CONTROLLING THE THICKNESS OF EACH OF THE COATINGS, A DUPLEX COATING IS FORMED WHICH IMPARTS GOOD CORROSION RESISTANT AND GOOD LACQUER-ADHERENT PROPERTIES TO THE FERROUS METAL ARTICLE WITHOUT IMPARING THE FORMABILITY THEREOF, THEREBY MAKING THE ELECTROCHEMICAL TREATMENT AND THE RESULTANT PRODUCT PARTICULARLY SUITED FOR THE PRODUCTION OF TINLESS CAN STOCK.

United States Patent US. Cl. 204-41 6 Claims ABSTRACT OF THE DISCLOSURE A two-step electrochemical treatment of a ferrous metal article in which a coating of electrolytic chromium plate is deposited on a ferrous metal surface and a surface coating containing chromium as an oxide of chromium and metallic chromium is codeposited over the chromium plate from a chromic acid solution containing an electroplating additive, preferably a halide ion. By controlling the thickness of each of the coatings, a duplex coating is formed which imparts good corrosion resistant and good lacquer-adherent properties to the ferrous metal article without imparing the formability thereof; thereby making the electrochemical treatment and the resultant product particularly suited for the production of tinless can stock.

The present invention relates generally to a metal product which is electrolytically treated to improve the properties thereto, and more particularly to a ferrous metal article treated electrolytically to improve the surface properties thereof, and to an improved method of electrolytically treating a ferrous metal article to impart thereto improved surface properties, particularly good lacqueradherent properties and corrosion resistance.

The present invention is an improvement over the invention in electrolytic treatment of steel described in the Yonezaki et al. US. Pat. No. 3,296,100, the Asano et al. US. Pat. No. 3,295,936, the Uchida et al. US. Pat. No. 3,113,845, and the invention in the electrolytic treatment of steel and other metals described in the Kitamura US. Pat. No. 2,998,361 and in the Yonezaki et al. US. Pat. No. 3,257,295. While the products of the foregoing inventions for treating steel and other metal surfaces are an improvement in some respects over previous electrolytic and chemically treated metal products, there remains a need for providing a metal product, and particularly a ferrous metal product, which has a high degree of corrosion resistance and good lacquer-adherent properties, and which at the same time has good formability and is capable of being produced commercially in a shorter treatment time than is required by the prior art processes.

It is, therefore, an object of the present invention to provide an electrolytically treated ferrous metal product having improved properties.

It is a further object of the present invention to provide an electrolytically treated ferrous metal product having improved corrosion resistance and good lacquer adherence without impairing the drawing and forming properties thereof.

It is still another object of the present invention to provide an electrolytically treated sheet steel strip having good lacquer-adherent properties and improved corrosion resistance which can be economically produced on a continuous steel strip treating line.

It is also an object of the present invention to provide an improved process of electrolytically treating a ferrous metal product which imparts thereto improved chemical and physical properties and which can be economically produced on a continuous treating line.

3,567,599 Patented Mar. 2, 1971 It is a further object of the present invention to provide a process of treating a sheet steel product to provide thereon a two-layered or duplex coating wherein the thickness of either layer can be varied independently of the other.

Other objects of the present invention will be apparent to those skilled in the art from the following detailed description and claims to follow:

It has been discovered that a ferrous metal surface having good corrosion resistance and lacquer-adherent properties can be obtained by first providing the metal surface with a very thin electrolytically deposited metallic chromium plate which is of a thickness normally exhibiting poor corrosion resistance but good drawing properties and then further treating the surface electrolytically to provide directly over the thin chromium plate a second coating comprised of electrolytically codeposited oxide of chromium-metallic chromium which alone would not provide the desired improved properties. A ferrous metal article, such as a steel strip or panel, having a coating of metallic chromium sufliciently thin to have good drawing properties when applied directly on the steel surface exhibits a very limited degree of corrosion resistance (i.e., about 0.5 hour to red rust in the Cleveland condensing humidity test). However, the same steel panel having the same very thin layer of metallic chromium on its surface when further coated with an electrolytically codeposited oxide of chromium-metallic chromium film having a thickness below that required for acceptable corrosion resistance when applied directly on the steel, exhibits markedly improved corrosion resistance (i.e., at least 120 hours before the appearance of red rust in the Cleveland condensing humidity test). Thus, the duplex electrolytic coating of the present invention is capable of imparting significantly improved corrosion resistance without destroying the formability (i.e., drawing properties thereof). Where desired, heavier coatings of oxide of chromiummetallic chromium film over a wide range of thickness can be codeposited on the thin chrome plated steel sheet to provide even greater protection against corrosion without interfering with the drawing and forming properties thereof.

It has also been found that a product having good adherent properties for lacquer and other siccative coatings, and having in addition very high corrosion resistance, can be produced over a wide range of thickness of the electrolytic codeposited oxide of chromiummetallic chromium coating, when the latter is deposited from a chromic anhydride additive containing electrolyte bath directly over the film of metallic chromium. A steel strip having formed directly thereon a coating of codeposited oxide of chromium-metallic chromium which is sufliciently heavy to provide acceptable corrosion resistance exhibits very poor lacquer-adherent properties. However, when a coating of electrolytically codeposited oxide of chromium-metallic chromium having a thickness up to about 10 mg./ft. total chromium is applied over metallic chromium plate on the surface of a steel strip, the lacquer adherence of the chrome plated steel remains high and the corrosion resistance in the Cleveland constant condensing humidity chamber is very substantially improved. For example, by using a coating solution of the herein described Example 1 for codepositing an oxide of chromium-metallic chromium coating and with an electrolytic treatment time of between about one second at amperes/ft. (100 coulombs per square foot) and two seconds at 300 amperes/ft. (600 coulombs per square foot), oxide of chromium-metallic chromium coatings are formed on a chromium plated steel sheet or strip which exhibit improved corrosion resistance and very good peel seam strength (i.e., about 70 pounds). It is essential, however, that the electrolytic oxide of chromium-metallic chromium coating be codeposited directly over the thin chromium plate on the steel strip in order to provide both good lacquer-adherent properties and improved corrosion resistance without impairing the formability thereof. When the thickness of the electrolytically codeposited oxide of chromium-metallic chromium coating is maintained preferably within a range of from about 1 mg./ft. to about mg./ft. total chromium (16.4 mg. chromium per square foot corresponding to a film having a thickness of 1.0 microinch), the lacquer adherence of the treated product remains approximately at the high level of a chromium plated steel product before any chemical treatment and the corrosion resistance of the treated product is significantly improved. Where good lacquer adherence is not important, however, in electrochemically deposited coating of oxide of chromiummetallic chromium having a much greater thickness than 8 mg./ft. can be applied over the chrome plate and the corrosion resistance to moisture will be further increased.

The coating of metallic chromium which is applied directly on the surface of a ferrous metal in the present invention can be applied by any conventional chromium plating process. However, since the duplex coating of the present invention imparts greatly improved corrosion resistant properties over a single layer of electroplated metallic chromium, such as produced according to the Uchida et al., US. Pat. No. 3,113,845, with or without the Uchida et al, auxiliary nonelectrolytic treatment using a 23% solution of sodium dichromate or a 1% solution of chromic anhydride, it is possible and preferred in the present invention to use an electroplated metallic chromium coating of substantially reduced thickness in comparison with that which is required in the Uchida et a]. process. The thickness of the metallic chromium plate applied directly on the surface of the steel strip in the present invention generally has a thickness range of from about .05 microinch (about 1.0 mg. chromium per square foot) to about 1.0 microinch (about 16 mg. chromium per square foot), and preferably has a thickness of about 0.2-0.5 microinch (about 3 to 8 mg. Cr./ft. when producing tinless steel can stock. Metallic chromium plate within the foregoing range of thickness provides a highly receptive base for the subsequent electrolytically codeposited oxide of chromium-metallic chromium coating and exhibits good formability and workability properties. Also, because the process of the present invention permits the use of an ultra thin film of metallic chromium, the time required for chrome plating can be very substantially reduced to a plating time as short as /2 to 1 second, and the cost of applying the chromium plate to a steel strip can be substantially reduced. However, where processing time and space limitations are not important and for those applications where formability and workability of the treated steel product are of little concern, it is possible in the present invention to apply substantially thicker metallic chrome plating above 1.0 microinch in thickness without departing from the broad concept of the present invention.

In practicing the present invention, a metal article, such as a low carbon steel strip or panel of the type commonly used for tin plating, is first provided with a metallic chromium coating applied directly on the steel surface by immersing the cleaned and activated steel article as a cathode in a chromium electroplating bath having between about 200 and 500 grams chromic anhydride (CrO per liter, and preferably about 250 grams per liter of the chromic anhydride, and containing a conventional chrome plating bath additive, preferably about 2.5 grams per liter sulfuric acid (i.e., bath ratio of 100:1), while maintaining the bath at a temperature of about 100 F.-160 F. and passing an electric current through the bath at a current density of between about 180 and 300 amperes per square foot for a period of between Cir about .5 and 2.5 seconds, and preferably for about one second. The anode can be a conventional lead anode, e.g., an alloy comprised of 93% lead and 7% tin. Other chromium plating bath compositions and operating conditions suitable for depositing a like thin metallic chromium film directly on the steel surface can be used, if desired.

Thereafter, a thin coating of electrolytic oxide of chromium-metallic chromium is codeposited directly on the surface of the metallic chromium plated strip, after thoroughly rinsing the strip with water, by immersing the chromium plated strip as the cathode in an electrolyte solution comprised of between about 75 to 400 grams chromic anhydride per liter, and preferably about grams per liter and also having as an essential ingredient an electrolytic plating bath additive of the type used in chromium plating baths, and which preferably includes halogen acids or halogen salts, such as sodium chloride and sodium fluoride, for providing halogen ions in the bath. The preferred plating additive is sodium chloride used in an amount of between about .045 and about 0.36 gram per liter, and preferably using between about .15 and .2 gram sodium chloride per liter. The ratio on a weight basis of chromic anhydride to halide ion should be at least about 3700 to 1 and a maximum ratio of about 450 to 1. The optimum ratio is about 750 to 1.

When a relatively thin coating of oxide of chromiummetallic chromium is to be codeposited on the surface of the chromium plate, a current density of between about 50 to about 400 amperes/fif and preferably about 100 amperes per square foot, is applied for a period of about /2 to 3 seconds, and preferably for about 1 to 2 seconds, while the preferred electrolyte solution is maintained at a temperature of about F. Under the foregoing operating conditions the electrolytically codeposited film of oxide of chromium-metallic chromium generally has a thickness ranging between about 1 mg./sq. ft. and 10 mg./ft. total chromium. Heavier coatings can be applied, if desired, by increasing the current density above 400 amperes/fh The thickness of the oxide of chromium-metallic chromium film varies directly with the current density, the time of electrolysis, and the concentration of the bath additive (preferably halide ion), while varying inversely with the temperature of the electrolyte bath. The operating conditions can be selected which will co-deposit a surface film of oxide of chromium-metallic chromium of any desired thickness. And, while the thickness of the surface film of oxide of chromium-metallic chromium codeposited over the thin chrome plate can vary between 1 to 10 mg./ft. (as total chromium) and provide significantly improved corrosion resistance and good lacquer adherence, it has been found that a very satisfactory thickness for the surface film consisting of codeposited oxide of chromium-metallic chromium is about 3 to 8 mg./ft. (as total chromium), and the optimum is about 7 mg./ft. (as total chromium) where it is desired to provide a duplex coating having good lacquer adherence and good corrosion resistance, when producing tinless can stock. The electrolytic overcoating of oxide of chromium-metallic chromium which is electrolytically codeposited on the surface of the chromium plate is formed of about equal amounts of chromium as the oxide and chromium in the metallic form. A film having a thickness of 1 microinch corresponds to 16.4 mg. chromium per square foot.

When sodium dichromate is substituted for chromic anhydride in the foregoing electrolyte bath for applying the oxide containing coating on the chrome plate under the specified cathodic treating conditions, an altogether different surface coating is formed, since the resultant surface film deposited from a sodium dichromate containing bath does not contain any detectable amount of chromium in the form of an oxide.

Prior to applying the initial metallic chromium coating directly on the surface of a ferrous metal article, the metal article is thoroughly cleaned in accordance with good chromium plating practice. When the article consists of a steel strip or panel the steel is preferably subjected to a cathodic cleaning treatment in an aqueous caustic cleaning solution, such as a solution of sodium carbonate, sodium hydroxide and polyphosphates at a current density of about 50-300 amperes per square foot for at least five seconds while the solution is maintained at a temperature of preferably about 180 F. The steel is then rinsed, preferably with light brushing, and while in a wetted condition is passed to an activating bath, preferably comprising an unheated 1-20% by volume aqueous hydrochloric acid solution and maintained in the activating solution for about five seconds. If desired, in place of the hydrochloric acid activating bath, the steel surface can be subject to treatment in a -10% by volume sulfuric acid bath for about five seconds or sub jected to .anodic activation in a solution having a composition similar to the chromium plating bath and at a current density of from about 100-200 amperes/ft. for one to two seconds. The metal article is then thoroughly rinsed with water and passed while wet into a chromium plating electrolyte bath.

The following specific examples further illustrate a process of the present invention, but should not be construed to limit the invention to the particular ranges or operating conditions specified.

EXAMPLE 1 A panel of 55 pounds per base box (.006 inch thick), double reduced, bright black plate with its surface covered with rolling oil is cathodically cleaned in an aqueous caustic cleaning solution, such as 4 oz. per gallon of an alkaline cleaning solution (Pennsalt 86A, a product of the Pennsalt Company) which is composed primarily of .45 microinch. The panel is thoroughly rinsed with water and while wet is passed directly into a second electrolytic bath containing 100 grams per liter chromic anhydride (CrO and .15 gram per liter sodium chloride. With the chromium plated panel as the cathode, an electric current having a current density of 100 amperes per square foot is passed through the electrolyte solution for a period of one second while the bath is maintained at a temperature of about 140 F. to effect cathodic codeposition of a film of oxide of chromium-metallic chromium. The panel on being withdrawn from the econd electrolyte solution is thoroughly rinsed with water and dried. If desired, a protective oil or other protective coating can be applied to the dry panel.

The panel treated in the foregoing manner had a dull metallic surface appearance and exhibited good corrosion resistance when tested in the Cleveland constant condensing humidity cabinet (i.e., 120 hours, after which the test was terminated) and good lacquer-adherent properties when subjected to the American Can Company peel seam lacquer adherence test (i.e., 6570 pounds). When the surface overcoating of electrolytically codeposited oxide of chromium-metallic chromium was stripped from the panel and analyzed by X-ray diffraction, it was found to be composed of oxide of chromium and metallic chromium; and when analyzed chemically, it was shown that 1.6 mg./ft. chromium was in the oxide form and 1.3 mg./ft. chromium was in the metallic chromium form.

EXAMPLE 2 Following the procedure described in Example 1, additional panels of the black plate used in Example 1 were electrolytically treated in accordance with the operating conditions specified in the following Table I. The test results obtained on the treated panels are also given in Table I.

TABLE I Oxide of chromium- Ohromium plating metallic chromium conditions plating conditions 0 CH0 Test to Feel C.D. C.D. red seam Time (amp/ Temp. Time (amp/ Temp. rust test Type of coating (sec.) ftfl) F.) (sec.) ftfl) F.) (hrs.) Obs.)

Duplex coating (of present invention) 2. 5 180 *120 70 2. Cl 180 *120 70 l. 0 180 *120 65 1. 0 180 *120 70 1. 0 180 120 70 0. 5 180 120 70 Metallic chromium film (control) 5. 0 180 4. 0 85 2. 5 180 2. 0 75 1. 0 180 0. 5 75 0. 5 180 0. 5 75 Oxide of ehromiummetallic chromium film (control) 1. 5 100 140 1 70 2. 5 200 140 120 30 *Test terminated after 120 hrs.

sodium carbonate, sodlum hydroxlde and polyphosphates, EXAMPLE 3 by passing electric current through the cleaning solution and the panel as the cathode at a current density of about to 100 amperes per square foot for a period of about 10 to 20 seconds while the solution is at a temperature of between about 160 and 180 F. The surface of the panel is then thoroughly rinsed with water, preferably with light brushing, and passed while wet into an acid activation bath consisting of about 5% by volume hydrochloric acid for a period of five seconds while the bath is at room temperature (about 75 F). The panel is then thoroughly rinsed with Water and while still wet is passed into a metallic chromium plating bath containing 250 grams per liter chromic anhydride and 2.5 grams per liter sulfuric acid. With the metal plate as the cathode a current of about 200 amperes per square foot is passedthrough the bath for a period of two seconds and with the bath at a temperature of 110 F. to effect deposition of a layer of metallic chromium having a film thickness of about A continuous strip of double reduced, dry black plate .006 inch thick was treated on a continuous strip processing line by first passing the strip continuously through an alkaline cleaning solution having a temperature of about F. and a composition as specified in Example 1, wherein the strip was made cathodic While applying a current density of about 50-100 amperes per square foot for a period of about 20 seconds. The strip was then thoroughly rinsed with water and passed continuously through an acid activation bath consisting of about 10% by volume hydrochloric acid for a period of about five seconds with the bath at a temperature of about 75 F. After thoroughly rinsing with water at a temperature of about F., the strip was continuously passed through a metallic chromium plating bath containing 250 grams per liter of chromic anhydride and 2.5 grams per liter of sulfuric acid, and while the strip was made the cathode, a current of 200 amperes per square foot was passed therethrough for a period of two seconds to effect deposition of a layer of metallic chromium having a film thickness of about .45 microinch. After thoroughly rinsing with water, the strip was continuously passed directly into a second electrolyte bath at a temperature of about 110 F. containing 100 grams per liter chromic anhydride and .18 gram per liter sodium chloride, and with the strip as the cathode, an electric current was passed therethrough having a current density of 100 amperes per square foot for a period of two seconds to effect codepositing of a film of oxide of chromiummetallic chromium. The strip on being withdrawn from the second electrolyte bath was thoroughly rinsed with water and dried.

The strip treated in the foregoing manner had a metallic surface appearance and exhibited good corrosion resistance when tested in the Cleveland constant condensing humidity cabinet (i.e., 168 hours, after which the test was terminated) and good lacquer adherence properties when subjected to the American Can Company peel seam lacquer adherence test (i.e., 6570 pounds). When the surface coating of electrolytically codeposited oxide of chromium-metallic chromium was stripped from a portion of the treated strip and examined by X-ray diffraction, it was found to be composed of about equal proportions of oxide of chromium and metallic chromium (i.e., from about 60% Cr O -40% Cr to about 40%Cr O to 60% Cr).

While the present invention is in no way dependent on any theory explaining the mechanism by which the improved results are produced, it appears from extensive X-ray diffraction studies and microscopic examinations of the duplex coatings of the present invention that under the herein disclosed operating conditions two distinct films are formed, one consisting of a thin but relatively uniform porous metallic chrome plate formed directly on the ferrous metal in the chromium plating bath, and a second or outer film electrolytically deposited from the second chromic anhydride containing bath which contains both oxide of chromium and metallic chromium in about equal proportions with the other film being nonuniformly distributed over the surface of the chrome plate and having a large proportion thereof concentrated as islands in a random manner. It is believed that the random concentrations or islands are formed at the pore or imperfections in the chrome plate and provides a more uniform, impervious surface which resists corrosion without, however, covering all the chrome plate with a thick film which would interfere with the good lacquer adherence of the chromium plate. It is also possible that having metallic chromium in the surface film may further contribute to the improvement in the lacquer adherence of the resultant duplex coating. The fact that the outer coating of oxide of chromium-metallic chromium must not exceed a maximum thickness in order to avoid interfering with the inherent good lacquer adherence of the chromium plate, however, suggests that the underlying chrome plate exerts a definite lacqueradherent effect through the outer coating when the thickness of the outer coating is restricted to the herein specified limits.

Others may practice the invention in any of the numerous ways which are suggested to one skilled in the art by this disclosure, and all such practice of invention are considered to be a part hereof which fall within the scope of the appended claims.

Iclaim:

1. A process of electrolytically treating a ferrous metal to improve the corrosion resistance and lacquer adherence of a chromium plated ferrous metal which comprises; depositing a thin metallic chromium plate on said metal by cathodically electrolyzing said ferrous metal in an aqueous bath containing hexavalent chromium with said metallic chromium plate having a weight between about 1 and 16 milligrams chromium per square foot, and thereafter making said chromium plate cathodic in an aqueous electrolyte bath consisting essentially of between about and 400 grams per liter chromic anhydride and sodium chloride with said chromic anhydride and the chloride ion of said sodium chloride being present on a weight basis in a ratio of between about 3700 to 1 and 450 to l, applying to said bath a current density of from 50 to 400 amperes per square foot for a period of from about /2 to 3 seconds with said electrolyte bath being at a temperature between about F. to F. to form a film of metallic chromium and oxide of chromium having a weight between about 1 and 10 milligrams chromium per square foot.

2. A process as in claim 1, wherein said electrolyte bath contains 100 grams chromic anhydride per liter and said sodium chloride is present in said electrolyte bath in an amount between about 0.15 and 0.2 gram per liter.

3. A process as in claim 2, wherein a current density of about 100 amperes per square foot is applied for a period of about 2 seconds. i

4. A process as in claim 1, wherein said film of codeposited metallic chromium and oxide of chromium has about 50% by weight of the chromium in sai film as metallic chromium and about 50% by weight f the chromium in said film as oxide of chromium.

5. A process as in claim 4, wherein said film of codeposited metallic chromium and oxide of chromium is maintained at a weight of between about 3 and 8 milligrams chromium per square foot.

6. A process as in claim 1, wherein said ratio of chromic anhydride to chloride ion on a weight basis is about 750 to 1.

References Cited UNITED STATES PATENTS 3,475,295 10/1969 Smith et al. 20456 1,975,239 10/1934 Ungelenk et al. 20441X 2,746,915 5/1956 Giesker et al. 20456 3,113,845 12/1963 Uchida et al. 20451X 3,245,885 4/1966 Asano et al 20456X 3,257,295 6/1966 Yonezaki et a1 20456 3,288,691 11/1966 Yonezaki et al. 20456 3,296,100 l/1967 Yonezaki et al. 20441 3,337,431 8/1967 Kitamura et al 20456 OTHER REFERENCES Edwin J. Smith: Chromium Coated Steel for Container Applications, preprint presented at 75th General Meeting of American Iron and Steel Institute in New York, pp. 4-15, May 25, 1967.

A. Kenneth Graham: Electroplating Engineering Handbook, p.193 (1962).

GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 204-28, 56 

